6 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



has not been simultaneous everywhere). The requirement for 

 perfection in preparative techniques varies with the questions that 

 are being asked. If a strongly bonded, insoluble protoplasmic 

 structure is under investigation, it may survive the ordeal of 

 preparation and yield valuable information while a neighboring, 

 more delicate constituent is distorted or destroyed. Fibrous 

 organelles are likely to be particularly sturdy. 



Minor variations in temperature, osmotic pressure, pH, and 

 timing of fixation may affect different kinds of cells, or of structures 

 within cells, quite differently; hence the desirability of exploring 

 alternative fixing methods for each new material. The compo- 

 sition of the embedding medium is another significant variable ; the 

 most commonly and easily used plastic, methacrylate, occasionally 

 damages cells during the hardening process. Other media 

 (Vestopal, epoxy resins) are being substituted with increasing 

 success. 



The recent emphasis on thin sectioning has tended to obscure 

 the valuable results that may be achieved by other methods. 

 Particularly where orderly patterns are spread out over a cell, 

 thin sectioning exposes too minute an area to permit detection of 

 the overall pattern. In these cases, surface replicas may be 

 informative; methods of fragmentation or progressive dissolution 

 often yield isolated fragments or pellicle strips that are essential 

 for the accurate interpretation of sections. 



Non-specific staining of cells before or after sectioning is often 

 helpful in increasing contrast in the final image. Heavy-metal 

 solutions such as phosphotungstic acid, lead hydroxide, or uranyl 

 acetate are among those most commonly used. The development 

 of specific procedures for electron-microscope cytochemistry is 

 well under way; these have rarely been applied as yet to the 

 protozoa, since they always require a thorough preliminary 

 familiarity with normal morphology. 



Measurements made on electron micrographs are of inconsistent 

 value. Examples will appear in which one author reports the dia- 

 meter of a given sort of fibril, say, as 8 m/x, while another measures 

 what should be precisely the same structure as 15 m^. The most 

 commonly used methods of calibration of the microscope are 

 recognized to be approximate, and final measurements are subject 

 to considerable individual error. At this stage in protozoology, 



